The dark deeps of the ocean has always been mysterious because they are dark (of course) as well hard to visit and see what is down there. For example the Coelacanth, long thought extinct, lives down deep and was only discovered in 1938 as well the elusive giant squids of legend. A study of the occurrence of fishes in the ocean's deepest reaches (the hadal zone, below 20,000 feet) has provided evidence that some species of fishes are more numerous at such depths than experts had thought. The authors of the study, which is published in the July/August issue of BioScience, observed 10 to 20 snailfish congregating at a depth of 25,000 feet around a baited video lander in the Japan Trench. The observation period lasted only five hours, so the occurrence of so many snailfish was a surprise.

The hadal zone (from the Greek for "like Hades") is the term used for the deepest oceanic trenches. This zone is found from a depth of around 20,000 feet to the bottom of the ocean.

It is believed that most life at this depth is sustained by organic debris falling from the upper regions of the ocean or the chemical reactions around thermal vents. The lack of light and intense pressure create hostile living conditions, and few species are adapted to these conditions. As no sunlight reaches this layer of the ocean, deep sea creatures have adapted with reduced eyesight, having very large eyes for receiving only bioluminescent flashes. Most of the deep sea dwelling creatures lack any pigmentation since coloration is not useful in an environment with no light. Life is weird to those more used to other marine forms.

The habitats chosen by snailfish are as widely variable as their size; they are found in both shallow intertidal zones and at depths of abour 24,000 feet or more, in both cold and warm waters. Some live closely related lives with scallops or kelp forests. Other species are found on muddy or silty bottoms of continental slopes.

Observations at such extreme depths (marine life is much more abundant on continetal shelves and in shallower water where sunlight is more abundant) is technically demanding and therfore rare.

The researchers who conducted the new study, Toyonobu Fujii of the University of Aberdeen, United Kingdom, and four of his colleagues, used a free fall lander that made video recordings of an illuminated patch of the sea floor for one minute every five minutes.

This enabled the scientists to distinguish at least 10 individual fish and record their behavior, which was similar to the behavior of fishes observed in 1965 from a bathyscaphe at a depth of 7300 meters in the west Atlantic. The fishes observed by Fujii and colleagues fed on crustaceans that were attracted to the mackerel bait.

How fish can live so far deep and removed from normal food supplies has long been a vexing question. Fish are often reported as being caught at great depths in fishing nets but there is uncertainty about what depth the fish were caught at. Fujii and colleagues remark that "current understanding of the hadal environment is inadequate." They nonetheless suggest that fish populations may routinely occur far deeper than previously thought in ocean trenches.

The depths of the ocean are poorly understood and there may be more interaction than suspected from the shallow depths to the deep abysmal levels.